Water is used in many industrial processes for a variety of applications such as steam production, cooling, washing, diluting, scrubbing, etc. In oil recovery processes, increased efforts have been made to conserve water by maximizing the reuse of process water and hence reducing the amount of waste water being discharged as well as fresh water make-up, resulting in both economical and environmental benefits. However, re-using process water has its own challenges since generally the process water is contaminated in its initial use and requires additional treatment such as filtration, sedimentation, flocculation, evaporation or chemical treatment before it can be re-used. Treatment of the process water for re-use must in itself be efficient and economical, and its extent determined by its intended use.
One such treatment method is termed mechanical vapor compression (MVC) evaporation. A compressor is utilized to produce the pressure and temperature differential to drive the falling film exchanger to produce a high purity distilled water product and a concentrated brine product. The schematic in FIG. 14 depicts a typical prior art MVC Evaporator System.
Evaporators have been used extensively in the mining as well as the pulp and paper industry as means of concentrating solids into a brine and/or recovering water from waste streams. In these applications, the solid contaminants are generally soluble in water. However, the SAGD process, as a result of injecting steam into an underground reservoir that is recovered as hot water with the production fluids, can introduce contaminants in different concentrations or that are not normally present. Oil and water-soluble solids present in the reservoir may cause variances in produced water quality at any given time, which can lead to operating problems in standard evaporator designs.
In the SAGD industry, the produced water, recovered from the SAGD production fluids, and make-up water, added to account for losses, must be treated to remove various contaminants to meet boiler feed water specifications. The contaminants include water hardness, silica, minerals, and residual oil/bitumen. If the water hardness, silica, and minerals are not removed from the water prior to steam generation via a boiler, they will precipitate in the boiler causing reduced heat transfer, lower capacities, higher boiler tube temperatures, extended boiler outages for cleaning and repairs and ultimately failure of the boiler. If the residual oil/bitumen is not removed from the water prior to steam generation via the boiler, there will be foaming and fouling issues in the boiler drum and tubes, again leading to process upsets and shutdowns.
The majority of SAGD production facilities utilize hot or warm lime softening systems combined with Weak Acid Cation (“WAC”) ion exchange systems to treat produced and make-up water. However, this process does not produce a high quality boiler feed water and necessitates the use of Once Through Steam Generators (“OTSG”), which only partially boil the feed water (75-80%) to prevent scale deposition (by maintaining solids in solution in the un-boiled water). This leads to energy inefficiency and excessive water disposal rates. OTSGs are custom built for the oil sands industry making them very costly compared to conventional boilers.
Recently, some SAGD operators have adopted falling film evaporators that produce a high quality distilled water for boiler feed water, which has made it possible to shift to more conventional drum boilers. The combination of falling film evaporators and drum boilers results in much higher water recycle rates (“WRR”) in a SAGD facility. This is becoming an increasingly critical environmental consideration.
However, operating companies are finding that there are many shortcomings with the current industry practice and evaporator system designs in SAGD facilities. Improvements to the current state of falling film evaporator design for SAGD water treatment have focused on the five most problematic technical issues that have been observed in the field:                Prevent accumulation of hydrocarbons in the evaporator sump;        Ensure silica, calcium, and other water soluble contaminants are maintained in solution to prevent scaling on the evaporator heat transfer tubes;        Select materials of construction suitable to the environment, such as high levels of chlorides in the evaporator sump due to the use of non-potable saline make-up water, pH levels in the sump, or the need to concentrate the brine to maximize water recycling;        Minimize power consumption in a water treatment unit where all of the recovered water is evaporated and re-condensed; and        Minimize the possibility of liquid carryover into the compressors of designs with mechanical vapor compression.Control of Hydrocarbon Accumulation        
One unique shortcoming not addressed by the current designs is the tendency of residual oil (including hydrocarbons, heavy oil and SAGD emulsifiers/reverse emulsifiers) to accumulate in the evaporator sump. The typical designs withdraw a concentrated brine stream from the evaporator sump at the outlet of the evaporator recirculation pumps. Owing to its lower density, oil will tend to slowly build up on the surface of the water in the evaporator sump. To control accumulation of contaminants in the evaporator sump, a controlled volume of water is removed from the system at the discharge of the brine circulation pumps. However, oil that accumulates on the surface of the water in the evaporator sump cannot enter the brine recirculation pumps, since the pump suction line is drawn from the bottom of the evaporator sump. The accumulation of oil on the surface of the evaporator sump will lead to “foaming” events in the evaporator sump, fouling of heat exchange surfaces, and the need to shutdown the evaporator sump to withdraw accumulated oil. The need to shutdown the evaporator to deal with foaming events reduces the overall reliability of the SAGD plant and reduces the production volumes. One objective of the invention is to remove the oil that accumulates on the surface of the evaporator sump, on a continuous basis, to prevent the foaming effect.
Control of Water Soluble Contaminants
The operation of the evaporator is time and labor consuming and has to be highly controlled before, after, and during the operation. A typical control scheme for an evaporator consists of the following:                The blow-down flow set-point is changed by an operator in response to a lab analysis of the concentration of solutes in the evaporator sump, so the concentration of solutes (silica, chloride, etc.) is controlled manually;        The evaporator feed rate is adjusted automatically by a sump level controller in response to changes in sump level;        The compressor speed and/or guide vane position is adjusted in response to the level of water in the distillate tank;        The production rate of distillate water from the evaporator is changed slowly in response to the level of the downstream tank, and in extremes, the production rate is changed in response to the level of the feed tank;        Startup and shutdown of the evaporator is done manually significant time pressures on the operator, the mode changes, especially startups and the response time immediately after a trip/malfunction, are the most dangerous times in a process plant; and        Operators make manual adjustments to rates to manage the inventories in 1) the upstream produced water tank that feeds the evaporator and 2) the downstream boiler feed-water tank that holds the evaporator product distillate water.        
An objective of this invention is to provide a process control scheme that provides system control across a broad range of operating conditions with minimal need for operator intervention. The typical prior art control scheme requires significant operator intervention, both during changes in operating mode (startup, shutdown, etc.) and periodically during operation, with manual adjustments to both feed flow and blow-down flow. Improved control will reduce the staffing requirements without affecting risk or operating costs, and in fact can simultaneously reduce risk and staffing costs and increase operating efficiency.
Automatic control of sump composition allows the evaporator to maximize efficiency of water use or power, depending on which is the most effective constraint.
Yet another objective of the invention is to remove the oil that accumulates on the surface of the sump on a continuous basis to prevent the foaming effect.
Another objective of this invention is the coordinated control of the different processing units, which will eliminate the need for online surge tanks, yielding a reduction in capital and operating costs compared to other processes.
Another objective of this invention is to reduce risk of damage, injury, production loss, or environmental incident by reducing the operator workload at the most critical time.
Further and other objects of the invention will become apparent to one skilled in the art when considering the following summary of the invention and the more detailed description of the preferred embodiments illustrated herein.